Selective laser solidification apparatus and method

ABSTRACT

A method of selecting a scanning sequence of a laser beam in a selective laser solidification process, in which one or more objects are formed layer-by-layer by repeatedly depositing a layer of powder on a powder bed and scanning the laser beam over the deposited powder to selectively solidify at least part of the powder layers, includes determining an order in which areas should be scanned by: projecting a debris fallout zone that would be created when solidifying each area based on a gas flow direction of a gas flow passed over the powder bed; determining whether one or more other areas to be solidified fall within the debris fallout zone; and selecting to solidify the one or more other areas that fall within the debris fallout zone before solidifying the area from which the debris fallout zone has been projected.

This application is a Divisional Application of U.S. patent applicationSer. No. 14/179,021, filed Feb. 12, 2014. The disclosure of the priorapplication is hereby incorporated by reference in its entirety.

The present invention relates to selective laser solidification and inparticular to an improved selective laser melting process and apparatusin which an order in which objects or parts of objects are built isselected based on a direction of gas flow.

BACKGROUND

Additive manufacturing or rapid prototyping methods for producingobjects comprise layer-by-layer solidification of a material, such as ametal powder material, using a laser beam. A powder layer is depositedon a powder bed in a build chamber and a laser beam is scanned acrossportions of the powder layer that correspond to a cross-section of theobject being constructed. The laser beam melts or sinters the powder toform a solidified layer. After selective solidification of a layer, thepowder bed is lowered by a thickness of the newly solidified layer and afurther layer of powder is spread over the surface and solidified, asrequired. In a single build, more than one object can be built, theobjects spaced apart in the powder bed.

During the melting or sintering process, debris (e.g. condensate,unsolidified particles of powder etc) is produced within the buildchamber. It is known to introduce a gas flow through the build chamberin an attempt to remove debris from the chamber in the gas flow. Forexample, the M280 model of machine produced by EOS GmbH, Munich, Germanycomprises a series of gas outlet nozzles located to the rear of thepowder bed that pass a flow of gas to a series of exhaust vents that arelocated at the front of the powder bed. In this manner, a planar layerof gas flow is created at the surface of the powder bed. A similararrangement is provided in Renishaw's AM250 and AM125 machines, whereinapertures at either side of a powder bed provide substantially planargas flow across the powder bed. It has been found that debris can beblown from one section of an object to another section of the object orto another object. This can result in non-uniformity and increasedporosity of the solidified metal layers generated by the meltingprocess. In particular, debris blown across the powder bed can result inan increase in surface roughness such that pores are formed betweenadjacent layers. Non-uniformities in a build can result in an object notconforming to the desired design and damage the apparatus. Inparticular, a wiper blade is typically used to spread each powder layeracross the powder bed. Solidified structures that project out of thepowder bed can catch on and cause damage to the wiper blade. Damagedwiper blades may result in powder layers with ridges of powder.Accordingly, non-uniformities in a build may be a concern not just forthe layer being formed, but powder layers formed thereafter.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided aselective laser solidification apparatus, comprising; a powder bed ontowhich a powder layer can be deposited, a gas flow unit for passing aflow of gas over the powder bed along a gas flow direction, a laserscanning unit for scanning a laser beam over the powder layer toselectively solidify at least part of the powder layer to form one ormore objects and a processing unit for selecting a scanning sequence ofthe laser beam based on the gas flow direction.

The scanning sequence may be selected such that debris produced by thescan is carried away from areas of the powder layer which are yet to bescanned. In this way, these areas of powder are not disturbed andcontaminated by the debris ensuring that, when these areas aresolidified, the solidified layer is built to a desired, uniform height.For example, the processing unit may select to scan one area beforeanother area because the area is located downwind in the gas flowdirection of the other area. Debris produced in forming the upwind areamay be blown onto the already formed downwind area but this debris maybe removed by the wiper and, if not removed, covered by the next powderlayer to be remelted when forming the next cross-section. Accordingly,scanning areas in this order is less likely to result innon-uniformities in the build.

The one or more objects may be formed through the solidification ofseparate islands in at least one powder layer, the processing unitarranged to select an order in which islands are formed based upon therelative location of the islands in the at least one powder layer andthe gas flow direction.

The order in which islands are formed may be selected such that debrisproduced by forming an island is carried away from areas of the powderlayer in which islands are yet to be formed. In this way, these areas ofpowder are not disturbed and contaminated by the debris ensuring that,when these areas are solidified, the solidified layer is built to adesired, uniform height. For example, the processing unit may select toform at least part of an island before at least part of another islandbecause the at least part of the island is located downwind in the gasflow direction of at least part of the other island. Debris produced informing the upwind island may be blown onto the already formed downwindisland but this debris will be covered by the next powder layer and thedebris is likely to be remelted when forming the next cross-section.Accordingly, building the islands in this order is less likely to resultin non-uniformities in the build.

For islands wherein a first island is located wholly downwind of asecond island, the processing unit may be arranged to select to form thefirst island completely before forming the second island. However, if afirst island is located to at least partially surround a second islandsuch that parts of the first island are downwind and other parts of thefirst island are upwind of the second island, the processing unit may bearranged to select to form at least part of the second island in betweenforming the downwind and upwind parts of the first island.

The processing unit may determine an order in which areas should bescanned by projecting a debris fallout zone that would be created whensolidifying each area and determining whether one or more other areas tobe solidified fall within the debris fallout zone, the processing unitselecting to solidify the one or more other areas that fall within thedebris fallout zone before solidifying the area from which the debrisfallout zone has been projected. The debris fallout zone may beprojected as a pair of parallel lines extending in the gas flowdirection from outmost edges of the area in a direction perpendicular tothe gas flow direction. Such a debris fallout zone may be suitable whenthe gas flow is a laminar gas flow (Reynolds number less than 2000).However, it may also be desirable to project the debris fallout zone asdiverging lines at a slight angle to the gas flow direction to take intoaccount slight turbulence in the gas flow that may cause the debris tobe deposited beyond the outmost edges of the area in a directionperpendicular to the gas flow direction. More complex models of theregion over which debris may be deposited may be used requiring morecomplex projections of the debris fallout zone. For example, areas to besolidified that are located close to an edge of the powder bed may besubjected to more turbulent flow because these areas are closer to thesidewalls of a build chamber than areas located centrally in the powderbed.

The apparatus may comprise an interface for the user to identify the oneor more objects to be formed. The user may select the locations on abuild platform where the one or more objects are to be built.Alternatively, the processing unit may be arranged to select thelocations on a build platform for the one or more objects. The processormay select the location for one or more of the objects based upon thedebris fallout zone of an object whose location has already beenselected.

According to a second aspect of the invention there is provided a methodof selecting a scanning sequence of a laser beam in a selective lasersolidification process, in which one or more objects are formedlayer-by-layer by, repeatedly, depositing a layer of powder on a powderbed and scanning a laser beam over the deposited powder to selectivelysolidify at least part of the powder layers, wherein a gas flow ispassed over the powder bed in a gas flow direction, the methodcomprising selecting a scanning sequence of the laser beam based on thegas flow direction.

The method may be a computer-implemented method.

According to a third aspect of the invention there is provided a datacarrier having instructions stored thereon, the instructions, whenexecuted by a processor, cause the processor to carry out the method ofthe second aspect of the invention.

According to a fourth aspect of the invention there is providedapparatus for selecting a scanning sequence of a laser beam in aselective laser solidification process, in which one or more objects areformed layer-by-layer by, repeatedly, depositing a layer of powder on apowder bed and scanning a laser beam over the deposited powder toselectively solidify at least part of the powder layers, wherein a gasflow is passed over the powder bed in a gas flow direction, theapparatus comprising a processing unit, a display and a user inputdevice, the processing unit arranged to receive data on one or moreobjects, including a location of the objects on a build platform, causethe display to display an image of areas to be solidified, wherein adebris fallout zone is projected from each area, and receive a userinput from the user input device of an order in which the areas are tobe scanned.

According to a fifth aspect of the invention there is provided a datacarrier for selecting a scanning sequence of a laser beam in a selectivelaser solidification process, in which one or more objects are formedlayer-by-layer by, repeatedly, depositing a layer of powder on a powderbed and scanning a laser beam over the deposited powder to selectivelysolidify at least part of the powder layers, wherein a gas flow ispassed over the powder bed in a gas flow direction, the data carrierhaving instructions thereon, which, when executed by a processor, causethe processor to receive data on one or more objects, including alocation of the objects on a build platform, cause a display to displayan image of areas to be solidified, wherein a debris fallout zone isprojected from each area, and receive a user input from a user inputdevice of an order in which the areas are to be scanned.

The data carrier of the above aspects of the invention may be a suitablemedium for providing a machine with instructions such as non-transientdata carrier, for example a floppy disk, a CD ROM, a DVD ROM/RAM(including -R/-RW and +R/+RW), an HD DVD, a BIu Ray™ disc, a memory(such as a Memory Stick™, an SD card, a compact flash card, or thelike), a disc drive (such as a hard disk drive), a tape, anymagneto/optical storage, or a transient data carrier, such as a signalon a wire or fiber optic or a wireless signal, for example a signalssent over a wired or wireless network (such as an Internet download, anFTP transfer, or the like).

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, as examples only,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a laser solidification apparatus accordingto one embodiment of the invention;

FIG. 2 is a schematic view of the laser solidification apparatus fromanother side;

FIG. 3 is a flowchart showing the steps of a method according to theinvention;

FIG. 4 is a plan view of islands to be solidified on a build platform ofthe apparatus, wherein debris fallout zones have been projected; and

FIG. 5 is a plan view of islands to be solidified on a build platform ofthe apparatus, wherein debris fallout zones according to a differentembodiment of the invention have been projected.

DESCRIPTION OF EMBODIMENTS

Referring to FIGS. 1 and 2, a laser solidification apparatus accordingto an embodiment of the invention comprises a build platform 102 forsupporting an object 103 built by selective laser melting powder 104.The platform 102 can be lowered in the chamber 101 as successive layersof the object 103 are formed. Layers of powder 104 are formed as theobject 103 is built by dispensing apparatus 108 and a wiper 109. Forexample, the dispensing apparatus 108 may be apparatus as described inWO2010/007396. A laser module 105 generates a laser for melting thepowder 104, the laser directed as required by optical module 106 underthe control of a computer 130. The laser enters the build chamber via awindow 107.

An inlet 112 and outlet 110 are arranged for generating a gas flowacross the powder bed formed on the build platform 102. The inlet 112and outlet 110 are arranged to produce a laminar flow having a flowdirection from the inlet to the outlet, as indicated by arrows 118. Gasis re-circulated from the outlet 110 to the inlet 112 through a gasrecirculation loop 111. A pump 113 maintains the desired gas pressure atinlet 112 and openings 5, 6. A filter 114 is provided in therecirculation loop 111 to filter from the gas condensate that has becomeentrapped in the flow. It will be understood that more than one inlet112 may be provided in the build chamber 101. Furthermore, rather thanextending outside of the build chamber 101, the recirculation loop 111may be contained within the build chamber 101.

Computer 130 comprises a processor unit 131, memory 132, display 133,user input device 134, such as a keyboard, touch screen, etc, a dataconnection to modules of the laser sintering unit, such as opticalmodule 106 and laser module 105, and an external data connection 135.Stored on memory 132 is a computer program that instructs the processingunit to carry out the method described with reference to FIGS. 3 to 5.

Referring to FIG. 3, geometric data of objects to be built, such as inthe form of an STL file, are received 201 by the computer 130, forexample over the external data connection 135. The processing unit 131receives 202 information on the location of the objects on the buildplatform 102. This location information may already be defined in theSTL or the user may the select, using the user input device 135, whereeach object should be located on the build platform 102. For each layer,the processing unit 131 identifies areas of the layer that are to besolidified and determines 203 an order in which these areas should bescanned by the laser beam. An example of how this may be done is shownin FIG. 4.

FIG. 4 shows five separate areas (islands) 122 to 126 to be solidifiedfor a particular layer. For each island 122 to 126, the processing unitprojects a debris fallout zone 122 a to 126 a in a gas flow directionfrom the island. The processing unit 131 then determines, for eachisland 122 to 126, if any other island falls within the debris falloutzone. If so, the processing unit selects to form this other islandbefore forming the island for which the debris fallout zone wasdetermined. For example, in FIG. 4, islands 125 and 126 fall within thefallout zone of island 122 and therefore, are selected to be scannedbefore island 122. Island 126 also falls within the fallout zone ofisland 125 and therefore, should be formed before island 125.

Rather than restricting ordering of the build to a complete island, theprocessing unit 131 may be arranged to select to form, in betweenforming different parts of the island, at least part of another island.FIG. 4 illustrates two examples of this. In the first example, island123 is completely surrounded by island 124. Accordingly, island 124comprises parts that are both upwind and downwind of island 123. In sucha scenario, the processing unit 131 selects to process the part of theisland that is located downwind of island 123 before scanning island 123and then scans the part of the island 124 that is upwind of island 123.The part of island 124 that is neither upwind nor downwind of island 123may be scanned before or after island 123 and the selection of thescanning order of these parts may be based on other criteria, such asscan speed. The different parts of island 124 are illustrated by thedotted lines. In the second example, rather than scanning all of island125 after scanning island 126, the part of island 125 that is not upwindof island 126 may be scanned before island 126. There may be reasons forscanning part of island 125 ahead of island 126, such as to optimizescan speed, variations in material composition and/or focal position.

In this embodiment, the processing unit 131 carries out this process foreach layer. However, in another embodiment, rather than calculating ascanning order for each layer, it may be possible to determine an orderfor multiple layers from a single analysis. For example, a fallout zonecould be determined from a footprint of each object on the buildplatform 102, the order being determined based upon whether otherobjects fall within a debris fallout zone calculated based on thisfootprint. Even though for some layers the debris fallout zone may besmaller than that calculated from the footprint, such a method mayprovide a reasonable generalization that reduces the amount ofprocessing required in determining an order in which the parts should bebuilt.

The selected order of scanning the parts may be displayed to the userand the user may be able to change the order. The user can then activatethe build to cause the processing unit to control 204 the optical module106 and laser module 105 to scan the powder layers to form the islandsin the selected order.

In the embodiment shown in FIG. 4, the debris fallout zones areprojected by extending straight lines in the gas flow direction fromedges of the islands. However, other projections of the fallout zonescould be used. Two examples are shown in FIG. 5. For island 127, afallout zone 127 a is projected as diverging straight lines at a slightangle to the gas flow direction to take into account slight turbulencein the gas flow that may cause the debris to be deposited beyond theoutmost edges of the island in a direction perpendicular to the gas flowdirection. A similar principal is embodied by island 128 and falloutzone 128 a, where an initially curved border to the fallout zone is usedto model that locally debris may be thrown out by the impact of thelaser beam on the powder layer but further from the island the debris ismore likely to be carried away along a straighter path by the gas flow.

In a further embodiment, rather than the processing unit selecting theorder in which islands are scanned, a user may select an order in whichislands are built. This may be achieved by the processing unit 131causing the display 133 to display images similar to those shown inFIGS. 4 and 5 so that the user can select the order islands are scannedbased upon this visualisation of the fallout zones. The processing unit131 then receives user inputs from the user input device of the order inwhich islands should be scanned.

It will be understood that in the above description, the islands maycome together in earlier or later layers so as to form a single objector may remain separate so as to form one or more separate objects.

It will be understood that alterations and modifications may be made tothe invention without departing from the scope of the invention asdefined herein. For example, the invention could be applied to a singleisland, wherein it is desirable to scan a downwind part of the islandahead of scanning an upwind part of the island.

The invention claimed is:
 1. A method of selecting a scanning sequenceof a laser beam in a selective laser solidification process, in whichone or more objects are formed layer-by-layer by repeatedly depositing alayer of powder on a powder bed and scanning the laser beam over thedeposited powder to selectively solidify at least part of the powderlayers, the method comprising determining an order in which areas shouldbe scanned by: projecting a debris fallout zone that would be createdwhen solidifying each area based on a gas flow direction of a gas flowpassed over the powder bed; determining whether one or more other areasto be solidified fall within the debris fallout zone; and selecting tosolidify the one or more other areas that fall within the debris falloutzone before solidifying the area from which the debris fallout zone hasbeen projected.
 2. The method according to claim 1, further comprisingselecting the scanning sequence such that debris produced during a scanis carried away from areas of the powder layer which are yet to bescanned.
 3. The method according to claim 1, further comprisingselecting to scan an area before another area because the area islocated downwind of the other area in the gas flow direction.
 4. Themethod according to claim 1, wherein the one or more objects are formedthrough solidification of separate islands in at least one powder layer,the method further comprising selecting an order in which islands areformed based upon a relative location of the islands in the at least onepowder layer and the gas flow direction.
 5. The method according toclaim 4, further comprising selecting the order in which islands areformed such that debris produced by forming an island is carried awayfrom areas of the powder layer in which islands are yet to be formed. 6.The method according to claim 4, further comprising selecting to form atleast part of an island before at least part of another island becausethe at least part of the island is located downwind of at least part ofthe other island in the gas flow direction.
 7. The method according toclaim 4, further comprising, for islands wherein a first island islocated wholly downwind of a second island, selecting to form the firstisland completely before forming the second island.
 8. The methodaccording to claim 4, further comprising, for a first island at leastpartially surrounding a second island such that parts of the firstisland are downwind and other parts of the first island are upwind ofthe second island, selecting to form at least part of the second islandin between forming the downwind and upwind parts of the first island. 9.The method according to claim 1, wherein the method is carried out by acomputer.
 10. A non-transitory data carrier having instructions storedthereon, the instructions, when executed by a processor, causing theprocessor to carry out the method of claim
 1. 11. A method of selectinga scanning sequence of a laser beam in a selective laser solidificationprocess, in which one or more objects are formed layer-by-layer byrepeatedly depositing a layer of powder on a powder bed and scanning thelaser beam over the deposited powder to selectively solidify at leastpart of the powder layers, the method comprising determining an order inwhich areas should be scanned by: determining, based on a gas flowdirection of a gas flow passed over the powder bed, a debris effectedzone into which debris created by solidifying a first area would becarried by the gas flow; identifying at least one second area to besolidified that falls within the debris effected zone and is downstreamof the first area in the gas flow direction; and selecting to solidifythe at least one second area before solidifying the first area.
 12. Themethod according to claim 11, wherein boundaries of the debris effectedzone are defined by parallel lines projected across the powder bed inthe gas flow direction.
 13. The method according to claim 11, whereinboundaries of the debris effected zone are defined by diverging linesprojected across the powder bed in the gas flow direction.
 14. Themethod according to claim 11, further comprising determining the orderin which areas should be scanned by: determining, based on the gas flowdirection, a further debris effected zone into which debris created bysolidifying a third area would be carried by the gas flow; identifyingat least one fourth area to be solidified that falls within the furtherdebris effected zone and is downstream of the third area in the gas flowdirection; and selecting to solidify the at least one fourth area beforesolidifying the third area.
 15. The method according to claim 14,wherein the debris effected zone and the further debris effected zone donot overlap.
 16. A method of selecting a scanning sequence of a laserbeam in a selective laser solidification process, in which one or moreobjects are formed layer-by-layer by repeatedly depositing a layer ofpowder on a powder bed and scanning the laser beam over the depositedpowder to selectively solidify at least part of the powder layers, themethod comprising determining an order in which areas should be scannedby: determining, based on a gas flow direction of a gas flow passed overthe powder bed, a gas-borne debris zone into which debris created bysolidifying a first area would be carried by the gas flow; identifyingat least one second area to be solidified that falls within thegas-borne debris zone and is downstream of the first area in the gasflow direction; and selecting to solidify the at least one second areabefore solidifying the first area.